1. Field of the Invention
This invention relates to earth boring bits lubricated with a seal system that retains lubricant in the bearings of the bit while excluding ambient drilling fluid. Specifically, the invention relates to improvements in the pressure compensating systems of such bits that minimize the pressure differential across the seals.
2. Background Information
The commercially successful, contemporary earth boring bit has rotatable cutters supported upon cantilevered bearing shaft and a system to provide lubricant to the bearings. A seal ring is utilized to retain lubricant inside each bearing, the most prevalent seal and bearing system being that of Edward M. Galle, U.S. Pat. No. 3,397,928.
A compensator system is included in earth boring bits for the purpose of minimizing the pressure differential across the seals. An example of a successful compensating system may be seen in the patent of Stuart C. Millsapps, Jr., U.S. Pat. No. 4,276,946, which utilize a flexible membrane or bladder, to separate drilling fluid from the lubricant and induce the hydrostatic pressure of the drilling fluid upon the lubricant. Thus, lubricant pressure is equal to the hydrostatic pressure when the bit is in a static condition. During drilling, there is movement of the cutter on the shaft due to the clearances necessary for manufacture and assembly. This movement changes the volume of lubricant within the bearing and tends to move the seal in its groove.
George Edward Dolezal discloses in U.S. Pat. No. 4,014,595 a bit that achieves pressure compensation by providing sufficient displacement of the seal, preferably an o-ring, to accommodate changes in the lubricant volume caused by cone movement and temperature changes encountered during drilling. By minimizing the volume of lubricant in the system, seal displacement is minimized to produce pressure equalization or compensation across the seal without need for additional compensation. This simplifies the lubrication system and should have enhanced reliability.
Leon Berthal Stinson and Edward M. Galle in U.S. Pat. No. 4,019,785 disclose a bit which included a lubrication system that achieves internal bearing pressure control through utilization of the thermal expansion of the lubricant, seal displacement and a pressure relief valve. Since the temperature of the earth generally increases with depth from the surface, a drill bit experiences increasing temperature with increasing depth. The coefficient of thermal expansion of lubricant inside the bit is greater than that of the metal defining the walls of lubricant cavities. The resulting increases in pressure caused by temperature increases of the lubricant is relieved by a pressure relief means, which can eliminate the necessity for a flexible diaphram-type pressure compensator.
In my above co-pending application, I disclosed a bit which utilizes a conventional pressure compensator of the type shown in Stuart C. Millsapps' U.S. Pat. No. 4,276,946 in combination with a metal face or rigid ring seal having the ability to compensate for the rapid or dynamic changes in volume in the lubricant adjacent the seal as the cutter moves during drilling. As a consequence, the seal ring is protected from static and dynamic pressure changes.